1. Field of the Invention
The present invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing the image signal recorded on the recording medium.
2. Description of the Related Art
It is known that various television systems such as NTSC systems and PAL systems have conventionally been used in still image signal recording/reproducing systems of the type which are arranged to record a still image signal on a recording medium and to reproduce a still image signal recorded on the recording medium. In general, the still image signal recording/reproducing systems utilize a method including the steps of frequency-modulating image signals of a format according to such a television system and magnetically recording the modulated image signals on small magnetic disks called video floppy disks. The resolution represented by the image signal recorded by the above method is substantially equal to that realized by the current television systems. However, if an image reproduced by such a still image recording/reproducing system is printed out by means of a printer or the like, image quality, particularly resolution, is low as compared to the resolution of a typical silver salt photograph.
In recent years, new television systems, such as a high-definition television (HDTV) system, have been investigated and proposed. The HDTV system realizes the vertical resolution of approximately 1,000 scanning lines per picture and has a signal band which provides horizontal resolution corresponding to the vertical resolution. For this reason, as a still image signal recording/reproducing system corresponding to the HDTV system, it is desired to provide a system capable of recording and reproducing still image signals which assure image quality of the order of 1,000.times.1,000 pixels.
In such circumstances, a recording/reproduction method which makes it possible to record still image signals with image quality as high as 1,000.times.1,000 pixels while retaining compatibility with the recording format used in a conventional type of still image signal recording/reproducing system, has previously been proposed in U.S. patent application Ser. No. 334,305 filed on Apr. 27, 1989, Ser. No. 344,202 filed on Apr. 27, 1989, Ser. No. 345,411 filed on Apr. 28, 1989, Ser. No. 450,393 filed on Dec. 14, 1989, Ser. No. 457,275 filed on Dec. 27, 1989, Ser. No. 460,308 filed on Jan. 3, 1990 now U.S. Pat. No. 5,075,802 and Ser. No. 460,306 filed on Jan. 3, 1990. The above method and the conventional method will be hereinafter referred to as a "CHSV (Compatible High-Definition Still Video) method" and a "SV method", respectively.
The operation of recording a luminance signal according to the CHSV method will be explained below. FIG. 1 partly shows the sampling positions of each luminance signal Y on a picture, which luminance signals Y are recorded on a video floppy disk by the CHSV method. The luminance signals Y (or Y signals) are sub-sampled and recorded in such a manner that each line of sample points is alternately offset in relation to the next adjacent line of sample points. For reproduction, the alternating sample points are converted into a lattice-like matrix of sample points by means of interpolation. The number of sample points is 600 (=1,200/2) per line and 500 (=1,000/2) per column, and information representing all the sample points in one picture is recorded on a total of four tracks. Referring to FIG. 1, for example, information representing the sample points contained in each line A.sub.1, A.sub.2, . . . is recorded on one particular track of the video floppy disk, information representing the sample points contained in each line B.sub.1, B.sub.2, . . . is recorded on another track of the video floppy disk, and so on. Recording on each track is performed in accordance with the recording format of the SV method, and the basebands of the Y signal and a chrominance signal C (or C signal) are approximately 7 MHz and approximately 1 MHz, respectively.
The following is an explanation of the analog transmission of sampled values which is performed in the CHSV method. FIG. 2 is a block diagram showing a conceptual arrangement for effecting such analog transmission. As shown in FIG. 2, an input analog image signal is sampled with a sampling period T and supplied to a transmission path 20, where it is subjected to the following processing. The sampled signal is first band-limited by a low-pass filter (LPF) 10, frequency-modulated by a frequency modulation circuit 12, and then recorded on a magnetic disk 14. The signal reproduced from the magnetic disk 14 is frequency-demodulated by a frequency demodulation circuit 16. The output of the frequency demodulation circuit 16 is band-limited by the low-pass filter (LPF) 18, which in turn outputs the result. As described above, the circuits 10-18 constitute the transmission path 20 having LPF characteristics. Then, a switch 22 re-samples the output of the LPF 18 and provides a sampled output. In the above-described method for analog transmission of sampled values, the re-sampling performed by the switch 22 plays an important role for restoring the original signal.
For example, if the signal input to the transmission path 20 of FIG. 2 is the sampled-value signal shown in FIGS. 3(a), the transmission path 20 provides an output waveform such as that shown in FIG. 3(b) in accordance with the LPF characteristics of the transmission path 20. If the period and phase of re-sampling pulses for controlling the re-sampling operation of the switch 22 (refer to FIG. 3(c)) completely coincide with the period T and phase of the input signal, the original input sampled-value signal will be able to be restored as shown in FIG. 3(d). However if the re-sampling pulses are out of phase as shown in FIG. 3(e), the waveform appearing on the output side of the switch 22 will be as shown in FIG. 3(f), and no sampled-value signal is correctly restored, thereby causing ringing. If the sampling periods differ from each other, correct restoration is similarly impossible.
Accordingly, when an image signal is to be recorded and reproduced on the basis of the CHSV method, it is necessary that re-sampling for reproduction be performed using a re-sampling pulse of correct frequency (period) and correct phase. In order to effect complete transmission (recording/reproduction) of the sampled-value signal, it is necessary that the transmission path 20 have linear phase characteristics (a flat group delay) and that its amplitude have a symmetrical roll-off characteristic centered at a frequency f.sub.s (1/2 T).
In order to accurately re-sample recorded sample points on the reproducing side, it is necessary that time-base variations which may occur during recording or reproduction be corrected accurately. For this reason, on the recording side, a sine-wave signal (reference signal) of frequency 2.5 MHz is frequency-multiplexed with the signal band defined between a color-difference frequency-modulated signal band and a luminance frequency-modulated signal band, and the frequency-multiplexed signal is recorded on a recording medium. On the reproducing side, a PLL circuit generates a clock signal having the same time-base variation component as the reproduced reference signal, and the reproduced image signal is written into an image memory in accordance with the clock signal serving as an image-memory write clock signal. The image signal stored in the image memory is read out in accordance with an accurate read clock, whereby time-base correction is effected.
However, the above-described method has a number of problems. For example, as shown in FIG. 4, the reference signal for correction of the time base is multiplexed with the frequency band between the color-difference frequency-modulated signal band and the luminance frequency-modulated signal band. As illustrated, the reference signal is adjacent to both the luminance frequency-modulated signal band and the color-difference frequency-modulated signal band. Accordingly, the reference signal leaks into the restored luminance signal or color-difference signal to exert an adverse influence, such as moire, on the image signal, thereby deteriorating the image quality. Moreover, the problem of the leakage of a reference signal into a restored luminance signal is also experienced with black-and-white image signals, since the black-and-white image signals and color image signals follow the same recording and reproducing processes.